Junichi Ihara, head of the Foreign Ministry’s Asian and Oceanian Affairs Bureau, lodged a protest with a senior diplomat at the Chinese Embassy in Tokyo following the intrusion.

The four Chinese Coast Guard vessels and three other ships were spotted just outside of the territorial waters earlier Friday, and the four entered at around 10 a.m., according to the 11th Regional Coast Guard Headquarters in Naha, Okinawa Prefecture.

The Japan Coast Guard warned the ships to leave, and one of the intruders returned a similar warning in Chinese and Japanese.

Chinese ships were most recently spotted in Japanese waters Saturday near the Senkaku chain.